1. Field of the Invention
The present invention relates to an apparatus and method for controlling a communication system. More particularly, the present invention relates to an apparatus and a method for reordering data packets in a communication system.
2. Description of the Related Art
The next generation communication system is being developed to provide high-speed and large-capacity services to Mobile Stations (MSs). As an example of a next generation communication system, there is a High Speed Packet Access evolution (HSPA+) communication system.
The HSPA+ communication system includes a layer structure. In order to send data packets received from Layer 1 to Layer 2 while ensuring in-sequence delivery, the HSPA+ communication system performs a reordering on the data packets received in a Medium Access Control (MAC) layer, that is, the MAC-enhanced high speed (ehs) entity, and sends the reordered data packets to a Radio Link Control (RLC) layer. Herein, the reordering of the data packets is performed in a reordering block included in the MAC-ehs entity. When the data packets are not received sequentially, the received data packets are reordered to ensure the in-sequence delivery using a T1 timer.
In the HSPA+ communication system, the operations of the T1 timer are defined, and the descriptions thereof are as follows.    (1) Starting of T1 Timer
In the case where a Transmission Sequence Number (TSN) of the received data packet is greater than the Next Expected TSN (NET), when the T1 timer is not in an active state, the T1 timer starts and the T1_TSN is set to the TSN of the currently received data packet which is greater than the NET. Herein, the NET denotes the TSN of the data packet which is expected for the reordering block to receive immediately after the currently received data packet. Further, the T1_TSN denotes a variable which is managed only when the T1 timer is in the active state, and the T1_TSN is discarded when the T1 timer expires.
In the HSPA+ communication system, there is a requirement that only one T1 timer may be in the active state during a predetermined time period when the TSN of the received data packet is greater than the NET and the T1 timer is in the active state. Therefore, in accordance with the requirement, when one of the T1 timers is in the active state, another T1 timer does not start.    (2) Stopping of T1 Timer
Before the T1 timer expires, if the reordering block sends the data packet with the TSN identical to the T1_TSN to a reassembly entity included in the MAC-ehs entity, the T1 timer stops. That is, when the reordering block receives a missing data packet, the T1 timer stops.    (3) Expiring of T1 Timer
If the T1 TSN is greater than the NET when the T1 timer expires, the data packets with TSNs ranging from T1_TSN to NET are sent to the reassembly entity. Further, the received data packets from the data packet with the T1_TSN to the missing data packet are sent to the reassembly entity. Further, the NET is set to the TSN of a data packet which is not yet received.    (4) Restarting of T1 Timer
If the T1 timer stops or expires and there is a data packet which cannot be sent to the reassembly entity, the T1 timer restarts and the T1_TSN is set to the largest TSN among the TSNs of the data packets which cannot be sent to the reassembly entity. In this case, the T1_TSN has the same value as the RcvWindow_UpperEdge. Herein, the RcvWindow_UpperEdge refers to the upper edge of a receiving window.
As described above, in Layer 2 of the HSPA+ communication system, the T1 timer operates, that is, starts, stops, expires or restarts to ensure in-sequence delivery.
However, in the HSPA+ communication system, there are only definitions for starting, stopping, expiring and restarting of the T1 timer for the in-sequence delivery. Accordingly, there is a need for an operation for reordering data packets using the T1 timer.